1. Field of the Invention
The present invention relates to a molding material and a molded part, which contain a biodegradable resin cross-linking product powder or biodegradable resin composite powder, and a method for manufacturing them. In particular, it relates to a biodegradable resin molded part useful for overcoming, in particular, a disposal problem after using in the field where plastic molded parts, e.g., films, containers, cabinets, and other structures and components, are used, a molding material for producing the molded part, and a method for manufacturing them.
2. Description of the Background Art
With respect to petroleum synthetic polymer materials, which are used for many films and containers now, a disposal process thereof alone has raised concerns about various social problems, for example, the global warming due to heat and exhaust gases accompanying disposal through the use of heating, adverse influences on foods and health exerted by toxic substances in combustion gases and in residues after combustion, and reservation of landfills for disposal.
Biodegradable polymer materials, typified by starch and aliphatic polyesters, have been noted as materials for overcoming the above-described disposal problems of the petroleum synthetic polymer materials. The biodegradable polymer materials have a small amount of heat accompanying combustion as compared with that of the petroleum synthetic polymer materials and do not exert adverse influences on the global environment including an ecological system, for example, a cycle of degradation and recomposition can be maintained in the natural environment. Among the biodegradable polymer materials, aliphatic polyester resins have characteristics comparable in the strength and the workability to those of the petroleum synthetic polymer materials, and are materials which have been particularly noted in recent years. Among the aliphatic polyester resins, in particular, polylactic acid is formed from starch supplied from plants, and becomes very inexpensive because of cost reduction based on the mass production in recent years, as compared with the other biodegradable polymer materials. Therefore, many researches have been conducted on applications thereof.
However, biodegradable resins, e.g., starch, cellulose, derivatives thereof, and polylactic acid, are very hard and exhibit substantially no elongation and, thereby, have disadvantages that the absorption of deformation and impact is poor. On the other hand, biodegradable resins, e.g., polybutylene adipate terephthalate, polycaprolactone, and polybutylene succinate, are flexible, but has a disadvantage that the breaking strength is low. As described above, the biodegradable resins do not have both the flexibility and the strength in combination and, therefore, most of them are not easy to use in their natural condition.
In order to improve the characteristics of these biodegradable resins, attempts, which are technologies having been carried out for known plastic resins, to mix resins with each other or to composite resins and modifiers, e.g., plasticizers, have been actively pursued. For example, with respect to the polylactic acid, “ARAKAWA NEWS”, No. 326, pages 2-7, issued in July 2004, by Arakawa Chemical Industries, Ltd., describes that a biodegradable resin (that is, polylactic acid) is kneaded with a specific plasticizer to improve the hardness and the brittleness at a temperature lower than or equal to a glass transition temperature of 60° C. and increase the impact resistance to the level of the common plastic.
Japanese Patent Laying-Open No. 2003-313214 proposes that a biodegradable resin is cross-linked by using ionizing radiation to overcome the problem in that the strength is reduced at a glass transition temperature or higher because of excessive flexibility.
However, when these individual technologies are used alone, it is not possible to dissolve both the maintenance of the flexibility and the elongation at a glass transition temperature of the biodegradable resin or lower and the maintenance of the shape and the strength (that is, the heat resistance) simultaneously. That is, the method, which is described in “ARAKAWA NEWS”, No. 326, pages 2-7, issued in July 2004, by Arakawa Chemical Industries, Ltd., and in which the plasticizer is mixed simply to the biodegradable resin, merely lowers the glass transition temperature and, thereby, weaken the bonding force between molecules of the biodegradable resin. Therefore, cracking becomes hard to occur because deformation tends to occur, but the restoring force against deformation and impact cannot be imparted. In addition, a vitreous material becomes like clay, and the strength cannot be maintained. As is described in Japanese Patent Laying-Open No. 2003-313214, cross-linking of the biodegradable resin is useful for improving the maintainability of the shape and the strength at a glass transition temperature or higher. However, the thus produced cross-linked biodegradable resin product is hard and brittle, and therefore, does not exhibit the flexibility and the elongation.
The above-described individual technologies can be carried out alone. However, when they are combined, inhibition of cross-linking occurs due to materials composited, and problems occur in that cross-linking cannot be effected and the like. For example, even when the above-described technologies are merely combined, and a composition, in which the biodegradable resin is kneaded with the plasticizer, is cross-linked by application of ionizing radiation or the like, cross-linking does not completely proceed. The causes of the inhibition of cross-linking are believed to be, for example, that the plasticizer eliminates radicals generated by the ionizing radiation and, in addition, when the plasticizer is kneaded antecedently, the plasticizer enters between the molecules of the biodegradable resin so as to inhibit bonding of the biodegradable resin molecules with each other. In order to cross-link the biodegradable resin, the biodegradable resin molecules must be contacted and bonded with each other.
Furthermore, previously known molded parts of the biodegradable resins having a cross-linking structure have been produced by being cross-linked through irradiation after being molded into the shapes of final molded parts, as described in Japanese Patent Laying-Open No. 2003-313214. Therefore, excellent workability and excellent productivity are not exhibited. The cross-linking of the biodegradable resin through irradiation has an advantage that the heat resistance and the shape-maintaining property are improved. Conversely, the above-described cross-linking allows the thermoplasticity of the biodegradable resin to deteriorate. Therefore, the irradiation is heretofore carried out after the molding into the product is carried out, as described above.
However, in order to irradiate after the molding into a desired shape is carried out, it is required to introduce an expensive and high-administrative-cost irradiation facilities into a production site or carry the molded part to the place where the irradiation facilities are disposed. Therefore, expenses, time and effort, and the like are involved, and the workability and the productivity of the molded part are impaired significantly. As a result, an excessive production cost is required.
The present invention has been made in consideration of the above-described problems. Accordingly, it is an object of the present invention to produce efficiently a biodegradable resin molded part having the heat resistance in a wide temperature range not by effecting cross-linking through irradiation after the molding into a desired shape is carried out, but by making it possible that a cross-linked biodegradable resin is made into a powder-shaped or pellet-shaped molding material and the resulting molding material is molded into a desired shape by various molding methods, e.g., inflation molding and injection molding.